1. Technical Field
The present invention relates to methods and compositions for removing a filter cake in a wellbore after drilling, especially horizontal sections of extended-reach and maximum-reservoir-contact wells in both open and cased hole completions.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
During drilling operations, the invasion of drilling fluid and particles that are smaller than the pore size of the drilled-formations into the zone around the well bore is one of the major factors contributing to reducing the productivity in the permeable zone by physical and/or chemical change to formation rock or fluids. This impairment is commonly referred to as formation damage. The volume of materials infiltrating the formation has a significant consequence on the extent of formation damage.
Typically over the permeable formation, filter cake is formed intentionally during drilling operations to prevent fluid losses to the formation and to allow good circulation of drilling fluids from the bottom hole to the surface. The filter cake composition depends on a well-designed drilling fluids and additives. The filter cake should allow for minimum filtration, e.g., minimum passage of drilling fluids through the filter cake into the formation, prevent invasion of solids into the formation, and withstand high differential overbalance pressures. For this reason, the drilling mud program usually involves comprehensive testing procedures to determine filtration rate and for better understanding of filter cake properties including cake porosity, permeability, structure, and others. Such information leads to the formation of an effective filter cake with properties that minimize the formations damage.
Investigators have strived to obtain a better understanding of the microscopic structure of the filter cakes by using different techniques. Several devices have been used to study the structure of filter cake such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-Ray Fluorescence (XRF).
On the other hand, the generated filter cake has to be removable to maximize the well productivity. Thereby, good knowledge of filter cake structure and composition leads to successful selection of an effective and efficient process and recipe for removing the filter cake. This issue becomes more severe in the maximum-reservoir contact and extended-reach wells due to the heterogeneity of the filter cake properties in the horizontal section and the presence of the fine drilling cutting particles in the filter cake in high concentrations which also degrade the sealing properties of the cake and leads to deep invasion. This makes it critical for investigators to have an effective technique for obtaining a typical filter cake sample and for removing the filter cake from the well bore. A filter cake operation that suffers problem may lead to extra formation damage in the target hydrocarbon production formation on top of that already existing during drilling operation. This in turn will add to the challenges of casing the well. In one aspect, as a filter cake that is formed over a permeable formation can be designed to include a bridging agent and a polymeric suspending agent. Such additives may help prevent the invasion of drilling fluid and drilling solids into the formation during the drilling operation. The filter cake removal composition should not be too aggressive to avoid early failure which may cause low removal efficiency and loss of the removal composition. A filter cake removal solution/composition may contain acidic components which may corrode the metallic of surface of drilling, casing or completion equipment.
Several methods are employed to overcome the challenge. The most recent method is the employment of two stage filter cake removal as described in U.S. Patent Application Publications US20130146289, US20130146295 and PCT Patent Application Publication WO2013160334 (each incorporated herein by reference in its entirety) that includes an enzyme, and a mixture of hydrochloric acid and an organic acid to remove the filter cake generated by a manganese-tetra oxide-based drilling fluid. The two stage filter cake removal composition is used by first applying the enzyme in a first stage for about 1 to 4 days based on the filter cake composition, the enzyme concentration and its thermal stability. In a second stage a mixture of hydrochloric acid and an organic acid is applied to the filter cake in the target production formation for about 18 hours and at about 250° F. The method comprising two stages aims to remove the filter cake of target production zone over an extended reaction time to control fluid loss from the wellbore.
U.S. Pat. No. 6,569,814 and US2001/0036905 (each incorporated herein by reference in its entirety) discloses fluids and methods of removing the filter cake or cleaning the well after drilling that includes the use of the coiled tubing unit but may not be efficient towards long horizontal wells.
A filter cake removal recipe must therefore meet several important requirements in order to operate effectively and provide high removal efficiency. Uncontrolled fluid losses of treatment fluids during removing the filter cake is a problem associated with filter cake removal in long open hole laterals. Such losses may lead to large portions of the well bore being untreated in intervals and result in filter cake remaining in the well bore. In high contrast or dense filter cakes which have low permeability, this problem becomes more severe.
Extended-reach drilling in carbonate and sandstone reservoirs is being used due to several reasons such as offshore drilling, maximizing reservoir contact, increasing production rate, and for thin reservoir access. The well clean-up is a very difficult process in long lateral horizontal wells because drilling fluid invasion can cause substantial damage to the well and/or the formation. Typically a drilling rig takes several days to be rigged down from the well and during this period the damage to the well and/or to the formation may increase. Coiled tubing is conventionally required to place the clean-up fluids to remove the damage caused by drilling fluid which. Coiled tubing processes are costly and time consuming.
In some cases, the horizontal well length might reach 20,000 ft or more. Currently available well clean-up fluid formulations are not able to clean up the well after drilling. In addition, coiled tubing is needed after the rigging down process is completed. This means more time and more damage to the horizontal section. Long horizontal wells may benefit from very low reactive fluids to clean the well and delayed reaction until after placing the fluid in the well.
Therefore, a need exists for methods and compositions that can remain in contact with the filter cake for a period of time sufficient to degrade the filter cake and can divert treated fluid to different sections in long laterals.